1. MEASURES OF ATMOSPHERIC COMPOSITION

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1. MEASURES OF ATMOSPHERIC COMPOSITION
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WHAT IS THE ATMOSPHERE?

• Gaesous envelope surrounding the Earth
• Mixture of gases, also contains suspended solid
  and liquid particles (aerosols)
• Aerosol dispersed condensed phase suspended in
  a gas

Aerosols are the visible components of the
atmosphere
Pollution haze over East Coast
Dust off West Africa
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ATMOSPHERIC GASES ARE VISIBLE TOOIF YOU LOOK
IN THE UV OR IR
NO2 Columns Observed from the SCIAMACHY Satellite
Instrument
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WHAT ABOUT CLOUDS?
Clouds are made up of water droplets (1-100 mm),
much larger than typical aerosols (0.01-10 mm).
They are technically aerosols but have unique
properties and are in practice considered
separately.
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1.1 Mixing ratio or mole fraction CX mol mol-1
remains constant when air density changes e
robust measure of atmospheric composition
GAS MIXING RATIO (dry air) mol mol-1
Nitrogen (N2) 0.78
Oxygen (O2) 0.21
Argon (Ar) 0.0093
Carbon dioxide (CO2) 380x10-6
Neon (Ne) 18x10-6
Ozone (O3) (0.01-10)x10-6
Helium (He) 5.2x10-6
Methane (CH4) 1.7x10-6
Krypton (Kr) 1.1x10-6

• Air also contains variable H2O vapor (10-6-10-2
  mol mol-1) and aerosol particles
• Trace gas concentration units
• 1 ppmv 1x10-6 mol mol-1
• 1 ppbv 1x10-9 mol mol-1
• 1 pptv 1x10-12 mol mol-1

Trace gases
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ATMOSPHERIC CO2 INCREASE OVER PAST 1000 YEARS
Intergovernmental Panel on Climate Change (IPCC),
2007
Concentration units parts per million
(ppm) number of CO2 molecules per 106 molecules
of air CO2 CONCENTRATION IS MEASURED AS MIXING
RATIO
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ATMOSPHERIC CO2 TREND OVER PAST 25 YEARS
IPCC 2007
mmol mol-1 is the proper SI unit ppm, ppmv are
customary units
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EPA SURFACE OZONE AIR QUALITY STANDARD 8-hour
average of 0.08 ppmv not to be exceeded more than
3x/year
EPA air quality standard for surface ozone is 75
ppb
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1.2 Number density nX molecules cm-3

• Proper measure for
• reaction rates
• optical properties of atmosphere

• Proper measure for absorption or scattering of
  radiation by atmosphere

nX and CX are related by the ideal gas law
na air density Av Avogadros number P
pressure R Gas constant Av k kBoltzmann
cnst T temperature MX molecular mass of X
Also define the mass concentration (g cm-3)
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TUESDAYS STRATOSPHERIC OZONE LAYER
http//jwocky.gsfc.nasa.gov/
Method UV solar backscatter
l2
l1
Ozone layer
Scattering by Earth surface and atmosphere
Ozone absorption spectrum
1 Dobson Unit (DU) 0.01 mm ozone at STP
2.69x1016 molecules cm-2
THICKNESS OF OZONE LAYER IS MEASURED AS A COLUMN
CONCENTRATION
l1
l2
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ANNUAL MEAN PARTICULATE MATTER (PM)
CONCENTRATIONS AT U.S. SITES, 1995-2000EPA
particulate matter assessment document (NARSTO),
2003
PM2.5 (aerosol particles lt 2.5 mm diameter)
U.S. air quality standard PM2.5 15 mg m-3
(annual mean)
Red circles indicate sites in violation of the
standard
STANDARD IS EXPRESSED AS A MASS CONCENTRATION PER
UNIT VOLUME
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SPECIFIC ISSUES FOR AEROSOL CONCENTRATIONS

• A given aerosol particle is characterized by its
  size, shape, phase, and chemical composition
  large number of variables!
• Measures of aerosol concentrations must be given
  in some integral form, by summing over all
  particles present in a given air volume that have
  a certain property
• The aerosol size distribution must be treated as
  a continuous function

URBAN
Typical U.S. aerosol size distributions by volume
RURAL
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1.3 Partial pressure Px Pa
Daltons law
Proper measure for phase change (such as
condensation of water vapor)
Evaporation of liquid water from a pan

• No lid water molecules escape from pan to
  atmosphere (evaporation)

• Add a lid
• escaping water molecules collide on lid and
  return to surface collision rate measures PH2O
• eventually, flux escaping flux returning
• saturation (PH2O,SAT)
• cloud formation in atmoshere requires
• PH2O gt PH2O,SAT
• T k e PH2O,SAT k

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CLAUSIUS-CLAPEYRON EQUATION PH2O, SAT f(T)
A 6.11 hPa B - 5310 K To 273 K
PH2O,SAT (hPa)
T (K)
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SHORT QUESTIONS
1. Give a rough order of magnitude for the number
of molecules present in a typical 1 micrometer
aerosol particle. 2. Consider harmful UV
radiation for which the ozone layer has an
optical depth of 10. The ozone layer has thinned
by 6 since 1970, with a corresponding 6
decrease in optical depth. What is the resulting
percent increase in this UV radiation reaching
the surface of the Earth?3. In an atmosphere
with fixed mixing ratio of water vapor, what two
processes can cause an increase in relative
humidity?
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PHASE DIAGRAM FOR WATER
http//weather.unisys.com/
Latest Observation for Cambridge, MA (02138)
Site KBOS Time 12 PM EDT 23 SEP 08Temp 61 F
(16 C)Dewpt 46 F (7 C)Rel Hum 58Winds E at
10 kntPressure 1033.2 mb (30.52 in)
gas-liquid metastable equilibrium
Relative humidity () 100(PH2O/PH2O,SAT) Dew
point Temperature Td such that PH2O
PH2O,SAT(Td)
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RUNAWAY GREENHOUSE EFFECT ON VENUS
due to accumulation of water vapor from volcanic
outgassing early in its history
did not happen on Earth because farther from
Sun as water accumulated it reached saturation
and precipitated, forming the oceans
EARTH
VENUS
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WHY CAN YOU SEE YOUR BREATH ON COLD MORNINGS?
Draw mixing lines (dashed) to describe dilution
of your breath plume w/outside air
PH2O is plotted on linear scale to draw the
mixing lines
LIQ
Your breath 37oC, 100RH
ICE
cloud!
no cloud
warm outside air
GAS
cold outside air
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AIR POLLUTION HAZE
Views of Acadia National Park
http//www.hazecam.net/

moderately polluted day
clean day
Visibility is limited by high concentrations of
aerosol particles that have swollen to large
sizes due to high (but lt100) relative humidity
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RAOULTS LAW
solute molecules in green
water saturation vapor pressure over pure liquid
water surface
water saturation vapor pressure over aqueous
solution of water mixing ratio xH2O
An atmosphere of relative humidity RH can contain
at equilibrium aqueous solution particles of
water mixing ratio
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HOWEVER, AEROSOL PARTICLES MUST ALSO SATISFY
SOLUBILITY EQUILIBRIA
Consider an aqueous sea salt (NaCl) particle it
must satisfy
This requires
At lower RH, the particle is dry.
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UPTAKE OF WATER BY AEROSOLS HAZE
NaCl/H2O
Deliquescence RH depends on particle composition
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SHORT QUESTIONS

• What is the fractional increase in mass of
  water-soluble aerosol particles when relative
  humidity increases from 90 to 95? (assume that
  the aerosols are mainly water). Assuming that
  visibility degradation is proportional to the
  cross-sectional area of the particles, what is
  the effect of this mass increase on visibility?